1. Field of the Invention
The present invention relates to recovery of mercury from drainage, and more particularly to mercury recovery from waste water having been used in cleaning exhaust gas from an incinerator, for example.
2. Description of the Related Art
When waster matters burned in an incinerator include mercury-containing products such as dry cells, fluorescent lamps, thermometers and the like, the incinerator lets out exhaust gas containing noxious mercury vapor. Thus, when the waste gas from the incinerator is treated with cleaning water, mercury is mixed into the cleaning water. To prevent environmental pollution, it is necessary to remove mercury from such mercury-containing waste water. The recovered mercury may be reclaimed for use as a resource. Mercury is dissolved in such waste water in the forms of mercury chloride, mercury oxide and the like.
In a conventional method of recovering mercury from waste water after cleaning such exhaust gas, the mercury-containing waste water is heated and aerated in a treating tank, thereby vaporizing mercury in the waste water into mercury vapor. Subsequently, the mercury vapor is cooled to become liquid mercury which is collected by a gas-liquid separator. Exhaust gas resulting from the gas-liquid separation contains liquid vapor in a quantity corresponding to saturated vapor pressure. To recover the residual mercury, the exhaust gas after the gas-liquid separation is fed back to the waste water in the heating tank.
In another known method which is an improvement upon the above method, metallic tin (Sn) is fed into a treating tank as a reducing agent to reduce mercury ions in waste water to metallic mercury. The rest of the process is the same as the above method, by which the metallic mercury is vaporized and released in vapor state from the treating tank, to be cooled into liquid mercury which is recovered through a gas-liquid separator. In yet another known method, waste water is alkalinized in a treating tank and stannous chloride (SnCl.sub.2) is applied thereto as a reducing agent. Subsequently, mercury is recovered as in the foregoing methods.
However, these known methods of recovering mercury have the following disadvantages which remain to be overcome.
Generally, exhaust gas from an incinerator contains hydrogen chloride (HCl), sulfur dioxide (SO.sub.2) and other substances. Waste water after cleaning such exhaust gas is acidic with hydrogen chloride, sulfur dioxide and other substances dissolved therein. However, mercury and its compounds, whether in gaseous state or solid state, have a property readily soluble in an acidic solution. Unseparated mercury vapor fed from a gas-liquid separator into such waste water will readily be dissolved back in the water. Once mercury vapor is redissolved, it is difficult to remove mercury from the waste water.
There is an additional problem that the treating tank and piping are corroded by the acidic waste water resulting from gas cleaning. It is therefore necessary to neutralize the exhaust liquid after recovery of mercury.
On the other hand, it is difficult to reduce mercury chloride and remove it from a weakly acidic solution.
Further, tin has a weak reducing effect unless turned into a very strong acid. Consequently, mercury recovery in weakly acidic regions requires a long gas-liquid contact time.
The treating time may be shortened by turning waste water into a strong acid for reduction purposes. However, this would increase the chance of the treating tank and piping becoming corroded, and aggravate the problem of treating waste liquid after the mercury recovery.
If, on the other hand, the above methods are executed by alkalinizing the waste water, it will be difficult for mercury vapor to redissolve in the waste water. In addition, although tin becomes sulfur dioxide ions in the solution and shows a strong reducing power, mercury oxide and other mercury compounds are not readily soluble in an alkaline solution but tend to form deposits. Once mercury compounds are deposited, the reducing reaction tends to be retarded, making it difficult to reduce all the mercury compounds sufficiently.